Significance This work will advance understanding of eye movement mechanics and strabismus. improving models of ocular statics, thereby facilitating development of better diagnostic and treatment procedures for strabismus. In the near-term, it will increase the impact of intraoperative muscle force measurement on surgical practice, improve estimates of muscle forces, needed for office procedures such as botulinum toxin injection, and clarify the response of extraocular muscles to surgical stretching and relaxation, and to post-surgical healing. Aims and Methods An implantable extraocular muscle force transducer (EOMFT), developed in the current grant period and tested in monkey, will be used in new monkey and human studies. (1) To resolve the discrepancy between intraoperative forced ductions, which show startling hysteresis, and motoneuron recordings. which do not, we will use monkeys and human volunteers undergoing strabismus surgery, with an eye containing an EOMFT that is subjected to external forces or left unrestrained. We expect to show that hysteresis is largely an artifact of the non-physiologic relationship between innervation and length during forced duction. The EOMFT will yield physiologic muscle force data, useful in guiding surgery. These data would also be free of the problems concerning primary position length, globe translation. and variations in musculo-fascial coupling. which plague existing methods. (2) We will measure horizontal rectus forces in human strabismus surgical patients by EOMFT and forceps methods, with and without short-acting paralysis of each muscle. Based on these studies, we will develop corrections to existing office procedures for estimating muscle forces with only forceps, with special emphasis on estimating effectiveness of treatment of lateral rectus paresis with botulinum toxin injection into the medial rectus. (3) To study slow changes in muscle force thought to follow eye muscle surgery, we will measure changes in force over several months following experimental muscle manipulations in monkey. Our method may distinguish changes in elastic force, due to plastic creep, for instance from changes in contractile force, possibly due to proprioceptive feedback. (4) We will transfer the EOMFT technology to collaborating laboratories.